Modern smart grid infrastructure envisions extensive deployment of renewable generation, PHEV and controllable loads, conservation voltage control and other technologies. To enable these deployments, there is a need for increased sensing and enhanced situational awareness for which Phasor Measurement Units are integral. With the growing deployment of Phasor Measurement Units (PMU's) in the substation level, there is increasing research interest in distribution level PMU development, but also interest lowering the cost of high voltage PMU deployments.
PMU's include voltage and current transducers giving a time-stamped measurement of the respective waveforms from which the magnitude and phase of each waveform can be extracted. Measurements can be taken at the substation as well as the line level. Currently, only about 2000 voltage PMU's are deployed in the US mostly at high voltage substations. The typical cost of PMU installation per circuit is 100 K USD where 90-95% of the cost is due to construction cost. New installations will be much more costly since the sensor technology requires not just a substation upgrade, but a full substation installation of voltage transducers and PMU enabling hardware. Additionally, no technology currently exists to monitor voltage and current phasors along high and medium voltage transmission lines, without building a full substation monitoring station.
The main reason for these limitations is the fundamental sensing methods required for high accuracy sensing, which rely on substation installed CCVT technology to step down the voltage and monitor it accurately. An alternative technology is that of line mounted voltage and current transducers which can contain the transducer, signal processing, PMU functionality and wireless communication as a full turnkey solution.
The primary technology used for non-standard voltage transducers have typically been (1) electrostatic field and (2) capacitively coupled measurements. However, these methods do not reach the required accuracy of the PMU standard.
There has been some attention paid to line mounted capacitively coupled voltage transducers for high voltage applications. Previously work has introduced ‘body capacitive voltage measurement’, however a calibrated capacitor divider circuit is required for determining the voltage on the line. The device is a doughnut shaped conductive material with an identical charge amplifier circuit. In another attempt, a ‘body capacitive’ probe having a fixed size sphere, which hangs on the power line is introduced. They present the sensing circuitry to measure the accumulated charge on the device as well. Like other works, the calibration of the probe capacitance is done offline. Recent work published develops a capacitively coupled transducer, where a similar understanding of the capacitive coupling and propose methods to track or mitigate changes in probe capacitance are presented. Here, multiple conductors are used to mitigate the effect of nearby conductors. The results show a nominal voltage magnitude error of 1-12% with 1-5 minute averaging periods. In a further attempt, an algorithm is proposed to mitigate the nearby conductors and determine the height of the device from ground. A parametric model relating the unknown height of the device from ground is used along with long time captures to estimate the height of the device and the probe capacitance. Finally, the estimate of the probe capacitance is used to estimate the line voltage.
What is needed is method of actively calibrated line mounted capitative voltage transducers for low cost high and medium voltage applications.